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Distributed Systems CS 3850 Soufiane Noureddine Lectures MWF 14:00 – 14:50 (PE207D) Office Hours MW 11:00 – 12:00 (C520) PowerPoint Presentation
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Distributed Systems CS 3850 Soufiane Noureddine Lectures MWF 14:00 – 14:50 (PE207D) Office Hours MW 11:00 – 12:00 (C520) Introduction Chapter 1 Computer Networks

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slide1

Distributed SystemsCS 3850Soufiane NoureddineLecturesMWF 14:00 – 14:50 (PE207D)Office HoursMW 11:00 – 12:00 (C520)

slide2

Introduction

Chapter 1

slide3

Computer Networks

A collection of computers communicating through an underlying network is called a computer network (in contrast to a single-processor system)

Local Area Network: LAN

10 to 1000 Mb/sec

Wide Area Network: WAN

64 kbps to gigabits/sec

definition of a distributed system 1
Definition of a Distributed System (1)
  • A distributed system is:
  • A collection of independent computers that appears to its users as a single coherent system.
  • A. Tanenbaum
slide5

Definition of a Distributed System (2)

“You know you have one when the crash of a computer you never heard of stops you from getting any work done.”

L. Lamport

organization of a distributed system
Organization of a Distributed System

1.1

A distributed system organized as middleware.Note that the middleware layer extends over multiple machines.

slide7

Characteristics of a distributed System

 Differences between computers are hidden

 Communication between computers is hidden

 Internal organization of the system is hidden

 Single system image: interaction with the system is independent from location (and time)

 Ease of extension

 High availability

slide8

Examples of distributed Systems

  • Network of workstations + Pool of processors:
  • - Single file system (uniform naming scheme)
  • - Processors are allocated dynamically when needed (load sharing)
  • - System acts like a single-processor system
  • 2. Workflow systems
  • - Orders arrive dynamically (e.g. via laptops, cellular phones)
  • - System assigns orders to the corresponding departments and initiates the needed business processes and users are unaware of the internal flow of orders
  • - System acts like a centralized database
  • WWW
  • - No need to know where documents are stored (at least in theory)
  • - Accessing remote documents is like accessing local ones
slide9

Goals of distributed Systems

  • Connecting users and resources:
  •  consequence: communication and collaboration (e.g. joint editing)
  •  issue: security
  • 2. Openness
  • - Services should obey to standard rules specifying syntax and semantics
  • - Services are specified in general as interfaces in an interface definition language
  • - IDL includes rather syntax and no semantics
  • - Specification of interface should be completed and neutral (w.r.t. implementation)
  • - Openness promotes interoperability and portability
  • Transparency (see next slides)
  • Scalability (see next slides)
transparency in a distributed system
Transparency in a Distributed System

Different forms of transparency in a distributed system.

Degree of transparency: More transparency means less performance

In reality: Systems are transparent only to certain degree

slide11

Network

(e.g. Telecom)

Clients

Software Developers

e.g. Company

Network Provider

slide12

Network

(e.g. Telecom)

Clients

Software Developers

e.g. Company

Company A

Company B

Network Provider

slide13

Network

(e.g. Telecom)

Clients

Software Developers

e.g. Company

Company A

Company B

Network Provider

slide14

Network

(e.g. Telecom)

Clients

Software Developers

e.g. Company

Company A

Company B

Network Provider

slide15

Network

(e.g. Telecom)

Clients

Software Developers

e.g. Company

Company A

Company B

Network Provider

slide16

Network

(e.g. Telecom)

Clients

Software Developers

e.g. Company

Applications

Operating System & Hardware

Network Provider

slide17

Network

(e.g. Telecom)

Clients

Software Developers

e.g. Company

Applications

AEM: Availability Enhancing Middleware

Operating System & Hardware

Network Provider

scalability problems
Scalability Problems

A system is scalable when it is easy to extend without loss of performance.

  • Examples of scalability limitations.
  • Centralized solutions tend to be non-scalable
  • Decentralized solutions promote scalability
slide19

Decentralized Algorithms

  • No machine has complete information about the system state
  • Machines make decisions based only on local information
  • Failures of one machine does not ruin the whole algorithm
  • No assumption on the existence of a global time
scaling techniques 1
Scaling Techniques (1)
  • Ways to solve scalability problems:
  • Hide communication latencies (geographical scalability)
  • Use of distribution (in order to avoid bottlenecks)
  • Use of replication
  • Ad a)
    • Batch processing: Asynchronous communication instead of synchronous communication
    • Interactive applications: Reduce overall communication

Example: Move part of computation from server to client (e.g. Java applet)

slide21

Scaling Techniques (2)

1.4

  • The difference between letting:
  • a server or
  • a client check forms as they are being filled
scaling techniques 3
Scaling Techniques (3)

Ad b) Use of distribution

1.5

An example of dividing the DNS name space into zones.

Clientserver Z1: nl.vu.cs.fluit

Server Z1  Client: address of Z2

Clientserver Z2: vu.cs.fluit

slide23

Scaling Techniques (4)

  • Ad c) Use of replication
  • Replication raises:
    • Availability: in general the primary goal
    • Performance:
  • By balancing the load among replicas
  • By locating replicas close to users/clients
  • Example: Caching (e.g. browser cache for used WWW pages)
  • Main issue in connection with replication: consistency
slide24

Hardware Concepts

1.6

Different basic organizations and memories in distributed computer

systems

slide25

Multiprocessors (1)

1.7

A bus-based multiprocessor.

Issues:

Scalability

Cache consistency

slide26

Multiprocessors (2)

1.8

  • A crossbar switch: n2 switches!
  • An omega switching network: less switches
slide28

Software Concepts

  • An overview of
  • DOS (Distributed Operating Systems)
  • NOS (Network Operating Systems)
  • Middleware
slide29

Uniprocessor Operating Systems

1.11

Separating applications from operating system code through

a microkernel.

slide30

Multiprocessor Operating Systems (1)

monitor Counter {

private:

int count = 0;

public:

int value() { return count;}

void incr () { count = count + 1;}

void decr() { count = count – 1;}

}

A monitor to protect an integer against concurrent access.

slide31

Multiprocessor Operating Systems (2)

monitor Counter {

private:

int count = 0;

int blocked_procs = 0;

condition unblocked;

public:

int value () { return count;}

void incr () {

if (blocked_procs == 0)

count = count + 1;

else

signal (unblocked);

}

void decr() {

if (count ==0) {

blocked_procs = blocked_procs + 1;

wait (unblocked);

blocked_procs = blocked_procs – 1;

}

else

count = count – 1;

}

}

A monitor to protect an integer against concurrent access, but

blocking a process.

slide32

Multicomputer Operating Systems (1)

1.14

General structure of a multicomputer operating system

slide33

Multicomputer Operating Systems (2)

1.15

Alternatives for blocking and buffering in message passing.

slide34

Multicomputer Operating Systems (3)

Relation between blocking, buffering, and reliable communications.

slide35

Distributed Shared Memory Systems (1)

  • Pages of address space distributed among four machines
  • Situation after CPU 1 references page 10
  • Situation if page 10 is read only and replication is used
  • Replicating all pages:
  •  Coherence protocols:
  • a) strong (transparent)
  • b) weak (not transparent)
slide36

Distributed Shared Memory Systems (2)

Page size: small  more communication overhead

large  less communication, but false sharing may occur

1.18

False sharing of a page between two independent processes.

Problem with DSM: not as efficient as expected

slide37

Network Operating System - NOS (1)

1-19

General structure of a network operating system.

Main features: Independent operating systems

Services for accessing remote resources

slide38

Network Operating System (2)

1-20

Two clients and a server in a network operating system.

slide39

Network Operating System (3)

1.21

Different clients may mount the servers in different places.

slide40

NOS vs DOS (3)

Distributed operating system:

Fully transparent

For homogeneous systems  computers are not independent More secure, but less scalable and less open

Network operating system:

Not transparent

For heterogeneous system  computers are independent

Easier to extend (e.g. adding a new node in the Internet)

Both do not really qualify as a distributed system!!!

 Solution: Middleware atop of a NOS hiding heterogeneity and improving transparency

slide41

Positioning Middleware

1-22

General structure of a distributed system as middleware.

Middleware Services: rather a functionally complete set of services in order to hide heterogeneity, direct access to NOS is discouraged.

slide42

Middleware and Openness

1.23

In an open middleware-based distributed system, the protocols used by each middleware layer should be the same, as well as the interfaces they offer to applications.

 implementations of the middleware should not use the NOS-provided protocols

slide43

Comparison between Systems

A comparison between multiprocessor operating systems, multicomputer operating systems, network operating systems, and middleware-based distributed systems.

slide44

Clients and Servers

1.25

General interaction between a client and a server.

slide45

An Example Client and Server (1)

The header.h file used by the client and server.

slide47

An Example Client and Server (3)

1-27 b

A client using the server to copy a file.

slide48

Processing Level

1-28

The general organization of an Internet search engine into three different layers

slide49

Multitiered Architectures (1)

1-29

Alternative client-server organizations (a) – (e).

slide50

Multitiered Architectures (2)

1-30

An example of a server acting as a client.

slide51

Modern Architectures

1-31

An example of horizontal distribution of a Web service.